It makes perfect sense to me. (Feels incredibly relatable, in fact) Mitsuki is a pretty typical artist, being jerked around by feelings that feel bigger than she is. She started feeling like a fraud, because she was playing music that she didn't "feel" in order to fit into her band, because she was afraid to try and make it on her own. She already felt like a fraud. When Aya saw her playing, and she saw Aya's expression (of concern, but to Mitsuki it must've felt like disappointment,) it made her feel like it was confirmed, the jig was up, and it was just a matter of time before Aya dumped her for somebody "real". When somebody is depressed, actually depressed, they have the least charity for themselves. Depression lies, and that's where Mitsuki's been living, under the lies her depression was badgering her with. Aya adores her, though, honestly, and has been trying to wait for her to open up, move on, anything really, but she didn't want to push.

Mitsuki let her insecurities come out to Aki, though, and it hit Aki at a really bad time, so when Aki's friend snubbed him, he started spiraling too. When the kid went missing, maybe because of what Mitsuki said to him, well, a moment like that has a way of snapping a lot of things into focus. Mitsuki is actually really good in a crisis, and so that got her confidence back, so she played, because when that fight or flight response actually kicked in, she's in it to win it. When she played, from the heart, and saw how much Aya loved the way she actually played, she got overwhelmed with feels in the moment and proposed on impulse.

Because honestly, who could look at somebody who is looking at you like that and not want to spend the rest of your life making them look like that?

Very definitely. A large proportion of the people in my life have been artists of various kinds at various levels. Big emotions often lead to blurting. And of course our mangaka is an artist, who would have such associations and such insights . . .
Of course our central pair need to work on things. With humans problems never just go away, they just become more manageable with time/wisdom (if we’re lucky).
I hope that Oosawa-san realises that there’s an alcohol problem though; it’s been blunting her natural courage and resourcefulness. Koga-san is going to need her partner’s strength, because that one is definitely not out of the woods yet (and may always remain something of a woodland creature . . . )
. And of course I’m invested in the other characters with their own longings and such. Although I could imagine Arai-sensei might eventually want to move on to other projects, or indeed take a sabbatical, at some point.

I perceive this as that playing music allows Mitsuki to articulate something that has been bothering her—it just all boils up to the surface. This is the beginning of a new and necessary conversation, not a Hollywood ending.
Also, while I’m grateful for the comments in this community, and the perspectives they offer, I also rather dread how people are going to want to have a particular outcome, and get really angry about anything that diverges from that. And yet it is good for me to see contrary opinions, and I hope that such commenters can stick with the story and trust that Arai-sensei is at least going to be true to Arai-sensei’s feelings and sense of how it must go.

The stormy confusion of all the panels; and then the the simple, decisive clarity of the last one. That works for me.

I guess no-one here has much patience with other people’s mental health issues, then? OK, maybe I’m not being fair. Mitsuki does have issues, and her current avoidance of music might well be symbolic of some other kind of trauma, such as grief, if you want to read this as literature?
For myself, music is a huge part of my life, but it’s not always been plain sailing, to put it mildly, whether as a listener, a performer or as a composer. Music is clearly extremely vital to Aya, and it has to be a very specific kind of music, and in this manga her character has been defined by it from the start.
You feel what you feel, and of course you might want to express that. But for me I still have, so far, the patience to spend a little while each Sunday morning with first the Japanese and then the translated version of this. Of course I’ll be relieved when this arc is worked out, one way or another. I hope it will be something that teaches me, and nourishes me, but I’m grateful for what has often been a refreshing story, and one of the highlights of each week.
When it’s all compiled into a volume, my perception of the overall pacing will change, and this definitely dark phase might feel less insistently upsetting.
But there are things here that Mitsuki needs to work out. And indeed Aya, whose refuge in the bottle maybe needs some attention, not to mention whatever had her find it so necessary to put so much weight on particular music. In real life such things are rarely actually worked out, but just fall into place, across a lifetime. We want better than that from fiction, though.
I don’t think Arai-sensei has written herself into a corner, but I think she’s going to try to find an honest way to resolve her character’s issues; and that might take some time yet, even if we all just want Narita to fix it all, which was a bit of a cop-out early on, when we were all still charmed by the art style and the characters. We can all see the pieces of the puzzle laid out on the table, and we’ve maybe all come to the realisation that the Narita-fix hasn’t worked this time. There are some lovely stories waiting to be told there, but, yes, the 4 pages per week format is tough on not just the story and its readers, but likely on Arai-sensei too. Not to mention whatever she’s going through with fame, and whatever is happening with the anime adaptation.

For me the pervasive problem that the characters face is Mitsuki’s relationship with music. So this seems to be a hopeful development, since it forces Mitsuki to realise she has a stake in it, and that it is hurting her most cherished relationship. She could of course interpret Aya’s concealment of the concert date through the green (haha) spectacles of jealousy, but even if this is her first thought, surely her second thought would have to be one of sorrow, that Aya is walking on eggshells on the topic of music . . . ? That would certainly be a motivation for her to once more confront music, which is, after all, what initially brought the couple together, and what repaired their first substantial conflict. But I’m biased because music (of a very different sort) is vitally important to me.

2 new chapters, with only one more to go!
I was so impatient for this one that I eventually got it in Japanese via Bookwalker (I’ve been studying Japanese for a couple of years). I love these characters. There are still subtleties I’m missing that translation will be helpful with, but I can say that the mangaka did not disappoint.